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The small blocks didn't always have a harmonic damper retaining bolt. I don't think the crankshafts were drilled and tapped for the 7/16" NF bolt until the 350's came along in 1969.

So the 265's through 327's had a light press fit harmonic damper and an oil slinger just in front of the timing chain sprocket. If the harmonic damper wasn't driven on tight the slinger and timing chain sprocket could eventually loosen and wear or even shear the crank shaft key so it was decided to add the 7/16" NF bolt to keep things tight.


The 7/16" NF bolt is a grade 8 bolt but it was never intended to be used to bar an engine over as many people do and end up with broken bolts and the grief getting them out. So never use that bolt to bar your engine over. I have often used the 15/16" (socket size) nut on my alternator to bar my engine over providing the belt is tight enough.
 
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